The Urban Dictionary Mug

A Narc is someone who comes on to a popular website and lists slang words people in the "know" use, and make them available to people of authority (teachers, cops, parents, parole officers etc.) Then because these Narcs think they are clever, we all get busted.

"Those kids on that slangdefinition site are unknowingly NARC minions and giving away our slang!" said BC.

The Urban Dictionary Mug

Narc is a shortened form of Narcotic and may refer to hard drugs or, by extension to a police officer from the drugs squad.

NB:
This word is often confused with nark. Nark is British and Australian slang for
(i) noun: a police informer, or more widely, anyone who reports illegal or anti-social activities to any authorities.
(ii) verb: to annoy, to irritate,

"See, that guy over there? The one who looks like a druggie? He's undercover, he's a narc."

"That guy just asked me for money for a cup of coffee"
"Cup of coffee? He's a junky, he wants his narcs."